`paths.relativize()` does not handle the case where `path == start`

`paths.relativize("/foo/bar/baz", "/foo/bar/baz")` returns `"/foo/bar/baz"` when I would naively expect it to return an empty string.

Locate paths.relativize() in the skylib source and inspect any existing tests for path utilities. Reproduce the equal-path example, then add coverage for the expected empty-string result and run the relevant test target.

Written by the indexing model from the issue text.